Planetary gear trains are complex mechanisms that reduce, or occasionally increase, the rotational speed between two rotating shafts or rotors. The compactness of planetary gear trains makes them appealing for use in aircraft engines where space is at a premium.
The forces and torque transferred through a planetary gear train place stresses on the gear train components that may make them susceptible to breakage and wear. In practice, conditions may be less than ideal and place additional stresses on the gear components. For example the longitudinal axes of a planetary gear train's sun gear, planet carrier, and ring gear are ideally coaxial with the longitudinal axis of an external shaft that rotates the sun gear. Such perfect coaxial alignment, however, is rare due to numerous factors including imbalances in rotating hardware, manufacturing imperfections, and transient flexure of shafts and support frames due to aircraft maneuvers.
The resulting parallel and angular misalignments impose moments and forces on the gear teeth, the bearings which support the planet gears in their carrier, and the carrier itself. These imposed forces and moments may cause gear component wear and increase a likelihood that a component may break in service. Component breakage is undesirable in any application, but particularly so in an aircraft engine. Moreover, component wear necessitates inspections and part replacements which may render the engine and aircraft uneconomical to operate.
The risk of component breakage may be reduced by making the gear train components larger and therefore stronger. Increased size may also reduce wear by distributing the transmitted forces over correspondingly larger surfaces. However increased size offsets the compactness that makes planetary gear trains appealing for use in aircraft engines, and the corresponding weight increase is similarly undesirable. The use of high strength materials and wear resistant coatings can also be beneficial, but escalates the cost of the gear train and therefore does not diminish the desire to reduce wear.
Properly lubricating the components of the planetary gear trains is essential to increasing the life and reducing wear of the components. However, as the size of a fan section driven by the planetary gear train increases, the planetary gear train will rotate at a slower speed. When the planetary gear train rotates at slower speeds, conventional methods of lubricating the planetary gear train may not generate adequate forces to move the lubricant to all of the components that require lubrication. Therefore, there is a need to enhance lubrication of the components of the planetary gear train.